The present invention relates to the comminution and grinding arts. It finds particular application in comminuting one centimeter sized vulcanized rubber pellets to micron sized particles in a single pass and will be described with particular reference thereto. It is to be appreciated, however, that the invention may find other applications in the comminution or size reduction of other solid materials such as synthetic and natural elastomers, plastics, coal, resins, and the like.
Heretofore, it has been found that micron-sized vulcanized rubber particles could be added to virgin rubber without adversely affecting its properties. Various methods and apparatus have been implemented to reduce vulcanized rubber and other elastomeric materials to micron-size particles.
As rubber becomes warmer, it tends to become more gummy or sticky. Whereas, when rubber is cooled it tends to become more brittle. To facilitate the reduction of rubber to smaller sized particles, the rubber was commonly cooled to a stiffer or more brittle state.
In one common method for reducing rubber to small particles, larger pieces of rubber were cooled to a cryogenic temperature at which the rubber behaved analogous to a brittle solid. The cryogenically cooled, embrittled rubber particles were ground with conventional grinding techniques. However, during the grinding, the temperature of the particles was raised above the cryogenic temperatures. The warming required that the particles be re-frozen and reground one or more times to reach micron-size.
Others have used cutting and abrasion devices for reducing the particle size of rubber. However, significant increases in the temperature of rubber from the heat generated in comminution were found to revert the rubber to a gummy or sticky state. The warm, gummy rubber tended to clog the pores or cutting surfaces of the abrasion and cutting devices. The warm, gummy rubber further provided increased resistance to moving comminution surfaces. The increased power required to move the abrading and cutting surfaces through the warm, gummy rubber tended to overload the comminution apparatus. Moreover, rubber is a combustible material and the hazard of combustion increases with increased temperature. The elevated temperatures generated in abrading and cutting tended to degrade the processed rubber material rendering it more combustible. Accordingly, the rubber material and the comminuting apparatus were commonly cooled. Efforts were made to maintain the temperature of the rubber to be comminuted and the abrading or cutting surfaces as cool as economically feasible.
The inventors herein have, contrary to the generally accepted wisdom, reduced the amount of cooling by increasing the temperature of a rubber and fluid slurry prior to comminution. The warmer temperatures are sufficient to reduce the viscosity of the carrier fluid without reverting the rubber to a gummy consistancy. In this manner, they have achieved an increase in the comminution rate of up to 25 percent and more.